An amplifier may include one or more stages. Each stage may include an amplifier that provides gain. As frequency increases, the gain that is provided by the amplifier tends to fall off, which limits the bandwidth of the amplifier. As operating frequencies of electronic computing devices increase, amplifiers having high bandwidth and gain and low noise have become increasingly important.
Miller compensation is a conventional frequency compensation technique that involves the movement of a dominant pole of a gain stage to a lower frequency by increasing the effective input capacitance of the gain stage. Miller compensation circuits include a Miller capacitance that exploits the Miller effect. When the Miller capacitance is connected in a feedback arrangement, the capacitance appears much larger at the input of the amplifier. While the dominant pole may be moved to a lower frequency using this approach, the gain and bandwidth of the system is still somewhat limited.
Referring now to FIGS. 1 and 2, an amplifier circuit 10 with Miller compensation is shown and includes first and second amplifiers 14 and 16, respectively. An output of the first amplifier 14 communicates with an input of the second amplifier 16. A first end of a Miller capacitance 18 communicates with the input of the second amplifier 16 and a second end of the Miller capacitance 18 communicates with the output of the second amplifier 16.
An input voltage of the amplifier circuit 10 is applied to an input of the first amplifier 14. An output voltage of the amplifier circuit 10 is referenced from the output of the second amplifier 16. As a result of the Miller compensation, the transconductance, gm, of the second amplifier 16 may be increased, which increases the bandwidth of the amplifier circuit 10. As can be seen in FIG. 2, the gain of the amplifier with Miller compensation has a 20 dB/decade slope.
Amplifiers may also be used with switched capacitive input signals. The switched capacitive input signals may be generated in analog to digital converters (ADCs), digital to analog converters (DACs), filters and/or other circuits. Traditional amplifiers such as those shown in FIG. 1 tend to have a difficult time providing sufficient gain and bandwidth at acceptable noise levels, particularly for switched capacitive input signals.